1. Field of the Invention
The present invention relates generally to methods for transmission of a digital data signal over partial response maximum likelihood (PRML) data channels. More particularly, the present invention relates to a system and method designed for use with a precoder such that the precoder produces codes with a Hamming distance greater than one to eliminate error events with a small distance at the output of the PRML channel. Still, more particularly, the present invention relates to an encoding system and method that produces at least a predetermined number of ones at the output of the channel to help derive a clock signal from the digital data signal.
2. Description of the Related Art
The use of PRML channels is well-known in the art as a method for transmitting data. There are several prior art methods for coding and detection in PRML channels. These prior art methods vary in their performance (error probability), and also vary in transmission rate.
The prior art often uses precoders to eliminate error propagation in PRML channels. For example, a 1/(1.sym.D) precoder over a 1-D channel produces non-zero channel output for each non-zero precoder input, and produces a zero channel output for each zero precoder input. In magnetic recording, a "1" channel input denotes magnetic polarization in one direction and a "0" channel input denotes magnetic polarization in the opposite direction. If a precoder is not used, then errors persist continually as the magnetic polarization directions are switched. Commonly, a precoder is used to eliminate this problem and exemplary precoders include: a 1/(1.sym.D) precoder for a 1-D channel; a 1/(1.sym.D) precoder for a (1-D)(1+D).sup.3 channel; a 1/(1.sym.D).sup.2 precoder for a 1-D.sup.2 channel; and 1/(1.sym.D.sup.4) precoder for a (1-D).sup.4 channel. However, such prior art precoding methods do not produce codes which have a Hamming distance greater than one for the channel. Therefore, there is a need for a precoder that produces codes having a Hamming distance greater than one at the input of a PRML channel to thereby improve performance and decrease error probability.
An additional problem with the prior art methods is their inability to provide sufficient clocking information at the output of the PRML channel. Clocking is the extrapolation of clock frequency and phase based on the received signal. Long sequences of consecutive zeros at the output of the PRML channel make clock detection difficult, and sometimes impossible. In contrast, a large number of ones or transitions at the output of the PRML channel can significantly improve clocking performance.
Accordingly, there exists a need for a system and method for producing many ones codes with a Hamming distance greater than one after precoding. Such codes would provide the dual advantages of improved channel performance and clock detection.